What big-time investors are snapping up and selling off

FORTUNE — It’s a particularly busy Friday on Wall Street, as investors and traders pour over quarterly filings that reveal which stocks in-the-know fund managers are loving (and not loving) and how much of it they’re buying (or selling). The 13F filings with the U.S. Securities Exchange Commission help Wall Street-ers divine a broader strategy behind what stocks are considered hot and not, so here’s where some of the market’s top soothsayers are placing their bets.

Warren Buffett’s Berkshire Hathaway brk.a went bullish on Verizon VZ, adding 11 million shares for a total value of $524 million. Buffett presciently sold off 10 million shares of General Motor GM last quarter, avoiding the recent stock drops in the wake of the company’s vehicle recalls and a hefty government fine. He also reduced his stake in oil and gas company Philips 66 by about 64% to 9.7 million shares.

John Paulson, best known for his $15 billion payday betting against sub-prime mortgages, put his money on Verizon as well, purchasing 8.7 million shares for a total of $416 million. His hedge-fund firm Paulson & Co., meanwhile, took a rosier view of GM and increased its stake by almost 115% to 4 million shares. One of Paulson’s biggest sell-offs was $375 million worth of Family Dollar FDO shares.

David Tepper’s Appaloosa Management was in lock-step with Paulson on GM, opting to boost his ownership in the Detroit, Mich.-based auto manufacturer by nearly 59% to 7.9 million shares. Tepper also put over $218 million down on Priceline PCLN and another $50 million toward Facebook FB shares. Meanwhile, he sold off all of his shares on five companies: EMC Corporation EMC, Foster Wheeler FWLT, Freeport-McMoRan Copper & Gold FCX, Hartford Financial Services Group HIG and Transocean RIG.

Barry Rosenstein’s GM outlook aligned with Buffett’s dim view as his Jana Partners sold off nearly all of its GM stake, cutting 8 million shares to just 7,100. Rosenstein opted to place his money in Groupon GRPN, increasing his stake of class A shares in the daily deals site by 9.8 million for a total of 40.8 million shares.

Bill Ackman, the famous and sometimes infamous activist investor, made very few changes in his Pershing Square Capital Management ownership, but his most notable move was buying up almost 29 million shares of Allergan AGN — or 9.7% of the company — in a bid to sell the drug maker to rival Valeant VRX. Ackman also bet on the rising demand for rental units by buying into two rental-focused REITs: Apartment Investment and Mangement Company AIV and Home Properties HME.

David Tepper: I’m nervous about the market

“There are times to make money,” says David Tepper, who runs Appaloosa Management. “This is a time to not lose money. I think it is a nervous time.”

On Wednesday, Tepper cautioned fellow fund managers and other attendees of this year’s SALT Investing Conference in Las Vegas. Tepper, who rarely talks about the market publicly, was recently named the highest paid hedge fund manager by Institutional Investor’s Alpha magazine. The magazine said he made $3.5 billion last year.

Tepper’s biggest concerns hinge on economic growth prospects and its effect on stock prices. He said his opinion would be different if the economy was growing at 4%. But he said that, even adjusting for the weather, the economy looks to be growing much more slowly than he expected. Indeed, U.S. GDP grew by 0.1% in the first quarter of 2014.

That’s a problem, Tepper says, because stocks on average are trading at 16 times next year’s expected earnings. That means investors are expecting relatively strong bottom lines. But if the economy is growing more slowly than expected, profits are likely to disappoint.

Tepper says he is also concerned about deflation, given the sluggish economic growth prospects. “If we have price pressure as well, then that’s really going to push down profits,” says Tepper.

Tepper says that while he is normally seen as a bullish investor, he currently has a portion of his money in cash. “I’m not saying go short,” Tepper says, which is the Wall Street term for betting against the market. “But don’t be super long either.”